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Show 164 DARWINISM OHAP. animals, n.nd presum<tbly from the same cause, too close intcr-brec: ling. . . Dean Herbert, who carried on cxpenmcnts With great c~rc and skill for many years, found numerous cases of hy~~·Jus which were perfectly fertile inter se. Crinum capcn~c, fcrt1hscd by three other species-C. pcdunculatu~, C. ca,nahculatum, or C. dcfixum-all very distinct from It, produced perfectly fertile hybrids; while other species less difl'crent in appearance were quite sterile with the sa.me C .. capensc. . All the species of the genus l-llppeastru~ produc~ ~ybrHl offsprinO' which arc invariably fertile. Lobcha syphyhtwt and L. fulgebns, two very distinct species, have produced a hybrid which has been uamed Lobelia speciosa, and which reproduces itself abundantly. Many of the beautiful pelargoniums of our greenhouses arc hybrids, such as P. ~gnescens ~ron~ a cr~s. between P. citrinodorum and P. fulgidum, whiCh 1s qmte fertile, and has become the parent of innumerable varieties of beautiful plants. All the varied species of C<1lceolaria, however different in appearance, intermix with the greatest rca.cliness, and the hybrids are all more or less fertile. But the most remarkable case is that of two species of Petunia, of which Dean Herbert says : "It is very rcmark<tblc that, although there is a great difference in the form of the flower, especially of the tube, of P. nyctanigenreflora and P. phmnicea the mules between them are not only fertile, but I have fouml them seed much more freely with me than either parent. .... From a pod of tho above-mentioned mule, to which no pollen but its own had access, I had a large batch of seedlings in which there was no variability or difference from itself ; and it is evident that the mule planted by itself, in a congenial climate, would reproduce itself as a species ; at lea~L as much deserving to be so considered as the various Calccolarias of different districts of South America.." 1 Darwin was informed by Mr. C. Noble that he raises stocks for grafting from a hybrid between Hhododendron ponticum and R. catawbiense, and that this hybrid seeds as freely as it is possible to imagine. He adds that horticulturists raise large beds of the same hybrid, and such alone are fairly treated; for, by insect agency, the several individuals arc freely 1 A maryllidacere, by the Hon. and Rev. William Herbert, p. 379. VII ON THE IN FERTILITY OF CROSSES 165 crossed with each other, and the injurious influence of close interbreeding is thus prevented. Had hybrids, when fairly treated, always gone on decreasing in fertility in each successive generation, as Gartner believed to be the case, the fact would have been notorious to nurserymen. I Cases of Ste'rility of Mong1·els. The reverse phenomenon to the fertility of hybrids, the sterility of mongrels or of the crosses between va1·ieties of the same species, is a comparatively rare one, yet some undoubted cases have occurred. Gartner, who believed in the absolute distinctness of species and varieties, had two varieties of maize-one dwarf with yellow seeds, the other taller with red seeds ; yet they never naturally crossed, and, when fertilised artificially, only a single head produced any seeds, and this one only five grains. Y ct these few seeds were fertile · so that in this case the first cross was almost sterile, though) the hybrid when at length produced was fertile. In like manner dissimilarly coloured varieties of Verbascum or mullein have 'been found by two distinct observers to be comparatively infertile. The two pimpernels (Armgallis arvcnsis and A. crerulca), classed by most botanists as varieties of one species, have been found, after repeated trials, to be perfectly sterile when eros. ed. . ~ o cases of this kind are recorded among animals ; but th1s 1~ not to be wondered at, when we consider how very few expenments have been made with natural varieties· while there is good reason for believing that domestic varieties are exceptionally fertile, partly because one of the conditions of domestication was fertility under changed condition. , and also because long continued clomestication is believed to have the cff'ect o~ increas~ng. fertility and eliminating whatever sterility may ex_1st. .Th1s IS shown by the fact tha,t, in m<my case., clomcstiC alllmals are descended from two or more distinct species. This is almost certainly the case with the doer a.nd probably with the hog, the ox, ancl the sheep ; yet Lhe v~'rious breeds are now all perfectly fertile, although we have every reason to suppose that there woulcl be some decrree of infertility if the several aboriginal . pecies were cro~sed together for the first time. 1 Origin of Species, p. 239. |
